A turbofan gas turbine engine nacelle intake is required to supply the fan of the turbofan gas turbine engine with favourably conditioned air during all operational conditions of the turbofan gas turbine engine, irrespective of the aircraft environment and aircraft attitude, whether the aircraft is in flight or on the ground. The nacelle intake may also be required to absorb noise generated by the gas turbine engine.
FIG. 1 is a partially cut away view of a turbofan gas turbine engine having, in axial flow series, an intake 12, a fan section 14, a compressor section 16, a combustion section 18, a turbine section 20 and an exhaust 22. The fan section 14 comprises a fan disc 24 carrying a plurality of circumferentially spaced radially extending fan blades 26. The fan disc 24 and fan blades 26 are surrounded by a fan casing 28. The fan casing 28 is mounted from the core casing 30 by a plurality of radially extending fan outlet guide vanes 32. The fan section 14 is driven by a turbine in the turbine section 20 via a shaft (not shown). The compressor section 16 is driven by a turbine in the turbine section 20 by a shaft (not shown). The whole of the turbofan gas turbine engine 10 is placed within a nacelle 34.
FIG. 2 is an enlarged vertical longitudinal cross-section, containing the engine axis X, through the intake of the turbofan gas turbine engine nacelle shown in FIG. 1. The nacelle 34 has an intake 12 at its upstream end and an exhaust 25 at its downstream end. The nacelle 34 intake 12 comprises, in flow series, a flared intake lip 36, an intake throat 38 and a diffuser 40 upstream of the fan section 14 of the turbofan gas turbine engine 10. The intake lip 36 forms a contraction for the supply of air to the intake throat 38. The diffuser 40 is arranged to diffuse the air from the intake throat 38 to the fan section 14. Generally the nacelle is constructed from a number of separate cowls, the front inlet cowl providing the intake lip and the diffuser.
The intake lip 36 has an inner surface and an outer surfaces. The highlight H is a closed loop running around the intake lip, and defines the boundary between the lip inner and outer surfaces. In FIG. 2 the highlight H is viewed edge on and is indicated by a solid line.
Travelling downstream from the highlight H along the inner wall of the nacelle on a longitudinal section, the downstream end of the intake lip 36 is reached when the tangent to the inner wall becomes 90° relative to tangent to the wall on the same section at the highlight. The position of the intake throat 38 can be taken to be the downstream end of the inner intake lip and the upstream end of the diffuser 40.
The diffuser 40 has a main diffuser section in which the inner wall curves in the airflow direction (as well as curving, of course, around the circumference of the diffuser) from the intake throat 38 towards the fan section 14. At the front flange of the fan casing 28, the downstream end of the curved inner wall then joins, with tangency matching, to the inner wall of the fan casing 28.
Optionally (and not shown in FIG. 2), the diffuser section may have at its downstream end a relatively short section in which, on the longitudinal section, the inner wall is straight (i.e. uncurved) in the airflow direction. This straight section of inner wall is tangency matched at its front end to the inner wall of the main diffuser section and at its rearward end to the inner wall of the fan casing.
A problem with known diffusers, however, is that they may not sufficiently reduce flow pressure asymmetries at the fan face. Such asymmetries can cause the fan to operate away from its optimum operating point and hence there is a loss of efficiency.